Sheet metal joining processes are widely used in many industries, such as the aerospace and automotive industries. Among these processes, resistance spot welding is the most common procedure used to join metal sheets because it has high process speed and is easily adopted in mass production lines. As these industries grow, the quality control of spot welds becomes an important issue for manufacturers eager to improve their output capacity and product quality.
The quality of the spot weld is affected by welding processes and the design of the joint. Many factors have to be taken into account, such as metallurgic reactions, thermal behaviors, chemical composition, condition of the base metal, welding conditions, and the welding equipment. Furthermore, the intricate relationship between these factors makes it more difficult to control the quality of spot welds. Numerous efforts have been made to improve weld quality through different approaches; nevertheless, most of them are not overall solutions due to the lack of adequate equipment and efficient algorithms to inspect these improvements.
The conventional strategy for spot weld quality control inspection usually consists of a weld current-resistance monitoring system to maintain consistent welding parameters, and an after weld, spot weld examination process, according to a standard set up by the American Welding Society for a particular industry. The spot weld examination standard typically includes visual inspection of the weld surface and destructive testing of collected weldment. To determine weld quality, visual inspection of the surface appearance and weld size are important indicators. Other important indicators, by destructive inspection, are weld size, penetration, strength, ductility, internal discontinuities, and sheet separation and expulsion. Weld consistency, assessed by monitoring welding parameters, is another important indicator. But these weld quality indicators are vague due to the insufficient quantified description. To apply these specifications in practical manufacturing cases, the indicators must be converted to quantified inspection standards. The Welding Handbook and the Resistance Welding Manual do indeed quantify these indicators, but even then spot weld quality control relies mainly on an on-line supervising unit to monitor welding parameters, on-line inspectors to perform visual inspection, and statistical sampling techniques for off-line destructive testing.
More importantly, the weld quality indicators are mostly for visual inspection and destructive testing, which are typically separately conducted. Thus, present weld quality control does not take into account the combined effect of those indicators. Furthermore, the true quality of the spot weld, i.e., its strength, is only presumed by off-line destructive sample tests. Unless every spot weld is examined, there is no certainty that the required strength has been met.
Acoustic methods are a commonly used non-destructive testing method that has been used for various inspection applications. Unlike other non-destructive testing methods, the acoustic method provides both surface and internal information. Moreover, the acoustic method allows deeper penetration into specimens and higher sensitivity to small discontinuities. Acoustic methods, however, are not flawless. The most significant limitations include the requirements of a propagating medium, or couplant fluid, which is required for acoustic wave propagation between the acoustic probe and the test specimen, and skillful operators for operating the devices and analyzing the acoustic information.
While the first limitation is typically overcome because the materials for joining in the automotive and aerospace industries are usually galvanized or coated and thus will not be damaged by any couplant fluid, the second limitation—the need for skillful operators—is much more significant. The on-line inspection of spot welds is very difficult because it is not economical to train every worker in the plant to be a tester/analyzer/operator.
More importantly, the acoustic method, by its very nature, limits the practicality of an on-line inspection. The acoustic method, unlike the optical or x-ray method that receives two-dimensional information through one process, has to go through point-to-point scanning procedures to obtain two-dimensional information. There are several ways to display acoustic information, and they can be categorized by the information obtained. The most common ones are A-, B-, and C-scans that can be selected to show the internal defects as required.
The A-scan, the simplest presentation, and widely used is conventional ultrasonic NDE devices, shows the amplitude of the echoes, or the reflection, as a function of time at a selected point on the work surface. The duration of time between different peaks represents the time needed for acoustic waves to travel between discontinuities.
The B-scan follows the same procedure as the A-scan, but repeats the signal-catching procedures while the probe scans along the straight line on the surface. Thus, an image of the cross-section of a component is built up. The measured amplitude is displayed as a colored dot on the monitor and its coordinant is defined by the position of the probe (X-coordinate) and the traveling time (Y-coordinate) of the acoustic pulse.
If the amplitude of a particular echo is monitored at each point on a certain depth of the workpiece, a C-scan can be performed. Measurements at each point are taken using two-dimensional scanning and electronic gate mechanisms that produce the plan for the level of the defect. This scan only gives the information at the preset depth of the electronic gate. While the C-scan provides the richest information, and is therefore more desirable for quality control purposes, it is also the most time consuming scan, and therefore difficult to perform on-line.
Conventional quality control devices for spot welding cannot perform on-line inspection of spot welds, nor can they provide feedback to the welding control system. In this way, the traditional quality control systems are similar to statistical welding parameter monitoring systems. While it is imperative to combine the idea of on-line quality inspection with closed-loop feedback control in a robust spot-welding control system, there is not an acoustic method capable of manipulating real-time control and on-line quality inspection.